Aluminum reflectors



United States Patent 3,312,535 ALUMINUM REFLEQTORS William A. Anderson,Verona, and Ronald Andrew, Pittsburgh, Pa., assignors to AluminumCompany of America, Pittsburgh, Pa, a corporation of Pennsylvania NoDrawing. Filed July 16, 1963, Ser. No. 295,517 8 Claims. (Cl. 29-183.5)

This invention relates to the production of high quality aluminum lightreflectors, that is, reflectors which have a high reflective efliciency.The surface of such reflectors is generally chemically orelectrolytically treated to provide a protective artificial transparentoxide film.

To obtain a maximum light reflectance from an aluminum surface it hasbeen found that it is necessary to employ high purity metal, usually notless than 99.8%. While such metal responds well to brightening and oxidecoating treatments, it suffers from diffi-c-ulties associated with grainsize. High purity metal under usual casting, working and annealingconditions develops relatively large or coarse grains. If such metal inthe form of annealed or lightly worked sheet, for example, issubsequently bent, formed or stretched, the surface acquires a roughenedappearance, especially where the deformation is most severe. Thisappearance is commonly referred to as orange peel and it detracts fromthe appearance of the reflective surface if not the reflectiveefiiciency. The defect is, of course, especially conspicuous on anotherwise smooth specularly reflective surface.

Conventional means of controlling the size of grains in such a highpurity metal have been ineffective or introduced other problems or addedto the cost of production to such an extent as to be consideredimpractical. The use of small amounts of the well-known grain refiningagents boron and titanium, for example, develop hard spots which giverise to streaks on the polished aluminum surface. The problem ofcontrolling grain size without adversely affecting the reflectivequality of the aluminum has received much attention over the years. Wehave found, after careful investigation, that the desired result can beachieved simply by the controlled addition of certain elements to thehigh purity aluminum.

It is an object of our invention to provide a highly reflective aluminumsurface free from surface defects.

Another object is to provide an aluminum light refleet-or which hasundergone deformation without the occurrence of the orange peel defect.

A further object is to provide a specularly reflective aluminum surfacehaving a fine grained structure.

These and other objects and advantages can be attained by our inventionwhich is predicated upon the discovery that the addition to aluminum ofa purity of not less than 99.8% of small amounts of manganese, copperand magnesium provides grain size control without any adverse effectsupon the light reflective quality of the aluminum. Although it has beenconsidered necessary in the past to limit the amount of unavoidableimpurities, including manganese, copper and magnesium, and otherelements in aluminum to achieve a high reflective quality, we have foundthat the addition of the three named elements within prescribed limitsdoes not detract from the reflective quality. More specifically it hasbeen discovered that by adding 0.l to 0.3% manganese, 0.05 to 0.2%copper and 0.1 to 0.25% magnesium to aluminum containing a maximum of0.2% impurities a small uniform grain size is assured in reflectors madeof the alloy. This combination of elements is essential to theattainment of the desired results, no one of them, or two of them, beingcapable of producing the desired reduction in grain size.

"ice

Referring to the individual elements, we have found that manganese,which is substantially insoluble in aluminum and forms a dispersoid,must be present in amounts of at least 0.1% to minimize grain growthduring hot Working and annealing operations. On the other hand, morethan 0.3% tends to decrease reflectance. Copper, an element soluble insolid aluminum, also has a grain refining effect, however, more than0.2% produces a yellowish tinge in the oxide coatings which isobjectionable. At least 0.05% copper is necessary in order to produce amarked reduction in grain size. The third element, magnesium, is alsosoluble in solid aluminum, and has a grain refining effect, the minimumamount needed to obtain this result being 0.1% While the upper limit is0.25%. Larger amounts diminish workability to the point of makingforming diflicult.

In respect to the impurities iron and silicon, not more than 0.1% ofeach should be permitted and preferably the maximum should be 0.05%each. Other impurities are also apt to be present but the amounts aregenerally too small to affect the working of the metal and itsreflecting quality. By using aluminum having a minimum purity of 99.8%,and preferably 99.9%, the impurity limits are not exceeded.

Reflectors are generally formed from sheet but other wrought forms thansheet can be employed depending upon the desired shape of the reflector.The sheet or other wrought form may be in the fully annealed condi tionor in a worked condition such that the fragments of grains are largeenough to cause orange peel or other defects when the product is formedor otherwise shaped and for this reason such fragments are referred toherein as grains. As a generality, the size of grain fragments is not aproblem where the reduction in cross section from that at which theproduct was anealed, or from the initial thickness of the stock, is morethan 50%.

It will be appreciated that While reflectors can be formed from sheet orother wrought products composed entirely of the alloy described above,that alloy may also be used as a cladding on another alloy havingdifferent properties. The same considerations relating to grain or grainfragment size apply to the cladding and hence in referring to reflectorsboth the clad and non-clad wrought products are contemplated.

To obtain the desired reflecting surface, the article shaped from sheetor other wrought form must be subjected to a sequence of surfacetreatments. Once the surface has been cleaned, mechanically polished orbuffed as required, it is subjected to conventional etching orbrightening treatments depending upon whether a diffuse or specularlyreflective surface is desired. An alkaline fluoride solution isexemplary of those Which produce a diffuse reflecting surface and anitric-phosphoric acid mixture as described in United States Patents2,650,157 and 2,729,551 illustrate a type which produces a brightenedsurface. Following such treatments the surface is subjected to aconventional oxide coating treatment that produces a transparent film.Electrolytic treatment in a sulfuric acid bath is an example of thosethat yield the desired result. Ordinarily the coating should not be morethan about 0.5 mil in thickness.

The sheet or other products are fabricated according to customarypractices of melting, casting, rolling or other working, and annealing.

The following examples illustrate the improvement gained by ourinvention over a well-known clad commercial product, identified as No.12 Reflector Sheet, the surface of which nominally consists of aluminum,0.04% copper and a maximum of 0.15% total of iron, silicon and otherimpurities. The alloy, representative of our invention, nominallyconsisted of 0.2% manganese, 0.1%

copper, 0.15% magnesium, a maximum of 0.15% total.

3 iron and silicon and balance aluminum. When cold rolled to 0.050 inchthick sheet and fully annealed, the commercial product had an averagegrain count of 56 grains per square millimeter while the sheet made fromour alloy had a grain count of 195 grains per square millimeter.

Samples of both annealed sheets were treated in a conventionalnitric-phosphoric acid solution to brighten the surface and thenanodically treated in a 15% sulfuric acid solution at a current densityof 12 amps per square foot and a voltage of 15. One group of samples wasremoved when the oxide coating acquired a thickness of 0.2 mil, a secondgroup was left in the solution until the coating became 0.3 mil inthickness while a third group was exposed long enough to develop acoating 0.4 mil in thickness. These samples were tested in aTaylor-Baumgartner reflectometer to determine total reflectance valueswith the following results.

It is evident from the foregoing that the combined presence ofmanganese, copper and magnesium in reflector grade aluminum has produceda finer grain size in annealed sheet than found in the commercialproduct when annealed under the same conditions. Also, that the additionof the three elements to the aluminum did not reduce the reflectancevalues as compared to those of the commercial material.

Having thus described our invention and certain embodiments thereof, weclaim:

1. An aluminum base alloy wrought article consisting of 0.1 to 0.3%manganese, 0.05 to 0.2% copper, 0.1 to 0.25% magnesium, a total of notover 0.2% impurities, and balance aluminum, said article beingcharacterized by a smaller grain size than the same article made fromaluminum containing no copper, magnesium and manganese except asunavoidable impurities and capable of developing as high a reflectancevalue by surface treatment as said aluminum containing no manganese,copper and magnesium.

2. A wrought article according to claim 1 wherein the alloy has amaximum of 0.1% impurities.

3. A light reflector having a surface composed of an aluminum base alloyconsisting of 0.1 to 0.3% manganese, 0.05 to 0.2% copper, 0.1 to 0.25%magnesium, a maximum of 0.2% all impurities and balance aluminum, thesurface of said reflector having a high uniform reflective qualitysurface and coated with a transparent artificial oxide film.

4. A light reflector according to claim 3 formed from a sheet of thedefined alloy.

5. A light reflector according to claim 3 formed from a sheet consistingof a base and cladding thereon composed of the defined alloy.

6. A light reflector according to claim 3 having a specularly reflectivesurface.

7. A light reflector according to claim 3 having a diffusely reflectivesurface.

8. A light reflector according to claim 3 wherein the oxide film is lessthan 0.5 mil in thickness.

References Cited by the Examiner UNITED STATES PATENTS 2,383,511 8/1945Reynolds 29l97.5 3,164,494 1/1965 English 75-139 X HYLAND BIZOT, PrimaryExaminer.

3. A LIGHT REFLECTOR HAVING A SURFACE COMPOUND OF AN ALUMINUM BASE ALLOYCONSISTING OF 0.1 TO 0.3% MANGANESE, 0.05 TO 0.2% COPPER, 0.1 TO 0.25%MANGNESIUM, A MAXIMUM OF 0.2% ALL IMPURITIES AND BALANCE ALUMINUM, THESURFACE OF SAID REFLECOR HAVING A HIGH UNIFORM REFLECTIVE QUALITYSURFACE AND COATED WITH A TRANSPARENT ARTIFICIAL OXIDE FILM.